626 



Yearbook^ of Agriculture 1949 



fastened in place than may always be 

 necessary for well-painted woodwork. 

 The weather, by alternately wetting 

 and drying the exposed surfaces of 

 boards more rapidly than the moisture 

 content can readjust itself within the 

 boards, sets up severe internal stresses 

 within the wood. The outcome is, suc- 

 cessively, a roughening of the wood 

 grain; wood checking; a tendency for 

 boards to cup, to withdraw nails, and 

 to split if they are unduly thin or short; 

 and a tearing loose of wood fibers from 

 the surface at such a rate that boards 

 lose as much as one-fourth inch of 

 thickness a century. For exposure with- 

 out painting, boards should not be 

 thinner than one-eighth their width 

 nor shorter than four times their 

 width; they should be kept firmly 

 fastened with hardware that with- 

 stands corrosion without painting. 



A coating of house paint on exposed 

 wood surfaces prevents weathering by 

 slowing down the entrance or exit of 

 moisture enough to keep nearly equal 

 moisture content from the center to 

 the surface of the boards. Internal 

 stresses are thus avoided, and the 

 boards stay flat and keep their smooth 

 surfaces. 



With white paint, which should be 

 renewed every 5 years, it may take 50 

 gallons of paint a century for each 

 1,000 square feet of surface to prevent 

 the weathering away of wood. It is 

 cheaper, of course, to omit the paint 

 and let the wood waste away slowly, 

 but the better appearance makes paint- 

 ing worth its moderate cost. 



Transparent finishes sometimes are 

 favored. The grain and color of freshly 

 cut and smoothly surfaced wood are 

 attractive enough to appeal to persons 

 who like the unusual. In consequence, 

 there is a demand for a transparent 

 protective coating that will serve, as 

 paint does, in preventing wood weath- 

 ering and yet will not conceal the 

 wood. 



Spar varnish is one way to do it, but 

 modernists seldom want the glossiness 

 of a varnish finish. For that reason, 

 linseed oil or certain proprietary oils 



or wood sealers, sometimes called log 

 oils, have become popular. 



The oils and sealers penetrate into 

 the surface of wood instead of over- 

 laying it with a coating as paint and 

 varnish do ; because the barrier against 

 the weather is imperfect when little or 

 no coating is interposed, the oils and 

 sealers furnish less protection than 

 varnish and much less than paint. Also, 

 because the transparent finishes are less 

 durable than paint, they must be re- 

 newed about once a year or oftener. 

 A century's protection for 1,000 square 

 feet of surface, therefore, may require 

 200 to 250 gallons of oil, sealer, or 

 varnish, whereas 50 gallons of paint 

 does the job more effectively. It is no 

 wonder our thrifty ancestors preferred 

 paint when they wanted smooth wood- 

 work! 



The transparent exterior finishes 

 have two further disadvantages. 



First, if they are allowed to go too 

 long before renewal, the wood begins 

 to turn gray from weathering. Once 

 that happens, renewal of finish must 

 begin with tedious scraping or sanding 

 away of the weathered wood to regain 

 a bright surface. 



Second, the transparent finishes are 

 readily attacked by fungi, which dis- 

 color the surfaces badly. The danger of 

 fungus attack, or mildew, can be re- 

 duced greatly by putting a suitable pre- 

 servative in the oil, sealer, or varnish. 

 Proprietary sealers and varnishes con- 

 taining a preservative are sold in paint 

 stores. When linseed oil is used, the pre- 

 servative, pentachlorophenol, can be 

 dissolved in it to the extent of 5 percent 

 by weight. If the wood contains sap- 

 wood, in which discoloring fungi grow 

 readily if the wood becomes damp, the 

 wood may well be treated with a com- 

 mercial water-repellent preservative 

 before the transparent finish is applied. 



Rough, unsurf aced wood, which may 

 be unduly expensive to paint, may 

 nevertheless be colored other than the 

 gray of weather-beaten wood. Shingle 

 stains are inexpensive kinds of paint. 

 They are made with pigments, linseed 

 oil, and much volatile thinner; they 



